1. Field
This patent document relates to an oscillator and a memory device including the same.
2. Description of the Related Art
Circuits such as memory devices, integrated-circuit (IC) chips, micro-controllers, and pump circuits use periodic signals for internal circuit operations. These periodic signals are generated by an oscillator and allow for synchronization of memory device operations.
The periodic signal of an oscillator has a period that varies according to the operating environment of the oscillator. Operating environments may change due to changes in temperature or operating voltages. To respond to these changes and optimize performance, the periodic signal may be adjusted to suit the given operating environment.
A memory device refresh operation is an example in which adjusting the oscillators periodic signal may improve performance. Memory cells of a memory device include a transistor serving as a switch and a capacitor for storing a charge (data). Whether a charge is stored in the capacitor of the memory cell, that is, whether the terminal voltage of the capacitor is high or low, indicates if the data point being stored is high (logic 1) or low (logic 2).
Since data is stored as an electric charge in a capacitor, no power is consumed in principle. However, since the initial electric charge stored in the capacitor diminishes due to leakage current caused by the PN Junction of an MOS transistor, or the like, data may be lost. In order to prevent such data loss, data stored in the memory cells must be read before the data is lost, and the capacitor must be recharged according to the information that was read. This recharging operation must be periodically repeated to retain stored data in a process that is known as a refresh operation.
Refresh operations include self-refresh operations that are performed in the memory device itself. Self-refresh operations may have optimized period signals that are adjusted according to charge retention time. For example, when the retention time is long (low temperature), the refresh period may be lengthened to reduce current consumption, and when the retention time is short (high temperature), the refresh period may be shortened to minimize the possibility of data loss.
The oscillator may be used to adjust the refresh period of the memory device. The oscillator may generate periodic signals that vary according to temperature. In order to prevent data loss and minimize power consumption, the period of the periodic signals needs to be precisely adjusted to suit the temperature in which the memory device is operating.